1. Field of the Invention
The present invention generally relates to an active device array substrate, in particular, to an active device array substrate having an electrostatic discharge (ESD) protection circuit.
2. Description of Related Art
Electrostatic discharge (ESD) is very common in our daily life. Because electrons have different affinities to different objects, charge transfer between two objects can be easily produced when the two objects are brought into contact and then separated, and accordingly electrostatic will be accumulated. Once a certain quantity of electrostatic has been accumulated in an object, a transient current may be produced when this object approaches or contacts another object at different electrical potential, and this phenomenon is referred as electrostatic discharge (ESD).
Generally speaking, an electronic product can be damaged by ESD easily when the electronic product is manufactured, assembled, transported, or even after the electronic product is purchased by a consumer. Thus, an electronic product has to be designed with ESD protection function in order to prolong the lifespan thereof. In particular, the circuits in a product fabricated through advanced semiconductor processes, such as an active device array substrate of a liquid crystal display (LCD), have small size. Thus, when the circuits are attacked by transient ESD of high voltage, the circuit inside the active device array substrate can be permanently damaged and accordingly the circuits are invalided.
In recent years, usually each pixel in an active device array substrate is divided into a main pixel and a sub pixel. The main pixel and the sub pixel are respectively controlled by two thin film transistors (TFT). Such pixels are referred as TFT-TFT type pixels. For providing stable storage capacitances to the pixels, a plurality of independent common lines has to be disposed correspondingly on the active device array substrate. However, these common lines are usually form as a large area of metal lines, and accordingly, electrostatic can be easily accumulated on these common lines to produce ESD. As a result, these common lines and devices or circuits around the common lines will be damaged by ESD.